The development of highly absorbent articles for use as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins is the subject of substantial commercial interest. The ability to provide high performance absorbent articles such as diapers has been contingent on the ability to develop relatively absorbent cores or structures that can acquire, distribute and store large quantities of discharged body liquids, in particular urine. These three functions can be accommodated by specific portions of the absorbent articles optimized for each. An acquisition material (or layer) is designed to take in liquid rapidly during a gush. The gush liquid is stabilized prior to being given up to the contiguous distribution material. The distribution material (or layer) has sufficient capillary pressure (described in more detail below) to pull liquid away from the acquisition member and distribute it throughout the absorbent article, often against the force of gravity to a height of 10-20 cm according to the size of the core. The storage member (or layer) has the highest capillary pressure and may contain hydrogel-forming absorbent polymers (HFAPs) to pull the liquid away from the distribution layer and store the liquid "permanently" away from the skin of the wearer.
Significant effort has been devoted towards the development of superior liquid acquisition and storage components. For example, U.S. Pat. No. 4,898,642 (Moore et al.) issued Feb. 6, 1990, U.S. Pat. No. 4,888,093 (Dean et al.) issued Dec. 19, 1989, U.S. Pat. No. 5,137,537 (Herron et al.), U.S. Pat. No. 5,217,445 (Young et al.), issued Jun. 8, 1993, and U.S. Pat. No. 4,822,453 (Dean et al.) describe curly, stiffened fibers that, when formed into low density webs, do not collapse when wet and retain their ability to acquire liquids at high rates as is experienced in a "gush" situation during urine voiding. Certain types of polymeric foams have been used in absorbent articles for the purpose of actually imbibing, wicking and/or retaining aqueous body liquids. See, for example, U.S. Pat. No. 3,563,243 (Lindquist), issued Feb. 6, 1971 (absorbent pad for diapers and the like where the primary absorbent is a hydrophilic polyurethane foam sheet); U.S. Pat. No. 4,554,297 (Dabi), issued Nov. 19, 1985 (body liquid absorbing cellular polymers that can be used in diapers or catamenial products); U.S. Pat. No. 4,740,520 (Garvey et al.), issued Apr. 26, 1988 (absorbent composite structure such as diapers, feminine care products and the like that contain sponge absorbents made from certain types of super-wicking, crosslinked polyurethane foams). U.S. Pat. No. 5,563,179 (Stone et al.) issued Oct. 8, 1996, describes hydrophilic absorbent foams useful for acquiring and distributing aqueous liquids in, e.g., absorbent cores. Similarly, various nonwoven materials have been proposed for liquid acquisition. Of key importance is the ability of these materials to acquire liquids repeatedly in use, to survive storage in a compressed state, and to release the acquired liquid to a subsequent liquid distribution or storage material.
The art is replete with examples of storage materials, such as "hydrocolloids" or "hydrogel-forming absorbent polymers" or superabsorbent polymers, summarized in "Water-Absorbent Polymers: A Patent Survey", Po, R. J. M. S.--Rev. Macromol. Chem. Phys. 1994, C34(4), 607-662. Such storage materials are usually blended with a fibrous web in varying proportions for use in absorbent cores. Other known storage materials include various hydrophilic foams, such as the emulsion-derived foams described in U.S. Pat. No. 5,387,207 (Dyer et al.) issued Feb. 7, 1995.
Comparatively much less development has been described in the literature regarding suitable distribution materials. Often, absorbent core designs contain no specific distribution material at all. Alternatively, the distribution function is combined with either the storage or acquisition function (as in U.S. Pat. No. 5,563,179, supra), which can result in somewhat compromised performance. Poor distribution with an absorbent core used, e.g., in a diaper can result in the accumulation of the acquired liquid in a relatively small part of the absorbent core, generally the crotch area. Here the aforementioned hydrocolloid or hydrogel-forming absorbent polymer ("HFAP") can convert the liquid into a gel. This has several undesirable effects even though the design is widely practiced in the art. Firstly, the accumulation of that volume of liquid in one area tends to distend the product, e.g., by extending the leg gathers in the case of a diaper, resulting in gaps between the product and the legs through which urine can leak. Secondly, the volume of liquid can be uncomfortable for the wearer. Thirdly, this concentration of urine can lead to undesirable effects on the skin which can result in localized dermatitis. Finally, this can also result in premature "gel blocking", as described in U.S. Pat. No. 5,599,335 (Goldman et al.), issued Feb. 4, 1997, with resultant inefficient utilization of the HFAP present in the product.
Effective distribution layers must be able to wick liquid vertically against the force of gravity to function properly in an absorbent core such as is found in a diaper. Vertical wicking capability derives primarily from the surface area per unit volume of the structure and its surface hydrophilicity. This can be measured as capillary absorption height (referred to herein as "CAH", defined infra). The CAH must be sufficient to acquire the aqueous liquids from any acquisition material used for temporary storage and wick the liquid to the remainder of the absorbent core, portions of which may be elevated 20 cm or more relative to the site of liquid insult by the wearer.
Another important property of a liquid distribution material is the ability to give up liquid to the storage components. This can be measured as capillary desorption height (referred to herein as "CDH", defined infra), which is always greater than the CAH. Classically, the CDH is at least about twice the CAH, the difference being referred to as capillary hysteresis.
Yet another important property is wicking speed. The distribution material must be able to wick the aqueous liquid to the height required within a reasonable period of time. In general, this time requirement is established by the rate or repeat insult of additional aqueous liquid in the loading zone. The distribution material should partition the aqueous liquid from the acquisition layer, wick the liquid to a required height, and partition the liquid to the storage layer substantially before the next insult occurs. A further requirement is that the material wick a sufficient volume of liquid so as to have a liquid flux adequate to substantially move the liquid out of the acquisition layer prior to the next liquid insult.
Yet another important property is amount of fluid that the material will absorb (on a g fluid per g material basis). Of particular note is the amount of fluid the material will absorb at a specific height (e.g., 15 cm) in the capillary sorption experiment. The distribution material ideally is able to absorb and move relatively large amounts of fluid per gram of material within the product.
The combination of sufficient CAH (for the height of the product), small CDH, and high wicking speed and flux has not been achieved with prior materials known in the art. Fiber webs can have good wicking speeds but are deficient in liquid flux because of their inherent low capacity (or free absorbent capacity) (due to the relatively high density or insufficient void volume). Thus, they cannot wick enough liquid over time to be adequate. Further, wherein a CAH of &gt;10 cm is needed, these fiber webs must be densified to provide the CAH needed, further reducing liquid flux (and void volume).
If made appropriately, open-celled hydrophilic polymeric foams can provide features of capillary liquid distribution required for use in high performance absorbent cores. Absorbent articles containing such foams can possess desirable wet integrity, can provide suitable fit throughout the entire period the article is worn, and can minimize changes in shape during use (e.g., uncontrolled swelling, bunching). In addition, absorbent articles containing such foam structures can be easier to manufacture on a commercial scale. For example, absorbent diaper cores can simply be stamped out from continuous foam sheets and can be designed to have considerably greater integrity and uniformity than absorbent fibrous webs. Such foams can also be prepared in any desired shape, or even formed into single-piece diapers.
Particularly suitable absorbent foams for absorbent products such as diapers have been made from High Internal Phase Emulsions (hereafter referred to as "HIPE"). See, for example, U.S. Pat. No. 5,260,345 (DesMarais et al.), issued Nov. 9, 1993 and U.S. Pat. No. 5,268,224 (DesMarais et al.), issued Dec. 7, 1993, both of which are incorporated herein by reference. These absorbent HIPE foams provide desirable liquid handling properties, including: (a) relatively good wicking and liquid distribution characteristics to transport the imbibed urine or other body liquid away from the initial impingement zone and into the unused balance of the foam structure to allow for subsequent gushes of liquid to be accommodated; and (b) a relatively high storage capacity with a relatively high liquid capacity under load, i.e. under compressive forces. These HIPE absorbent foams are also sufficiently flexible and soft so as to provide a high degree of comfort to the wearer of the absorbent article; some can be made relatively thin until subsequently wetted by the absorbed body liquid. See also U.S. Pat. No. 5,147,345 (Young et al.), issued Sep.15, 1992 and U.S. Pat. No. 5,318,554 (Young et al.), issued Jun. 7, 1994, which discloses absorbent cores having a liquid acquisition/distribution component that can be a hydrophilic, flexible, open-celled foam such as a melamine-formaldehyde foam (e.g., BASOTECT made by BASF), and a liquid storage/redistribution component that is a HIPE-based absorbent foam.
While these foam-based acquisition/distribution components afford rapid liquid acquisition and relatively efficient distribution and partitioning of liquid to other components of the absorbent core having higher absorption pressures, these foams are nevertheless compromise materials intended to accomplish at least two separate functions. Specifically, the microstructural morphology and mechanical strength of these materials has been optimized to meet two needs, rather than being specifically designed for only one purpose within the absorbent core.
Accordingly, it would be desirable to provide a material that: (1) is specifically designed as an efficient distribution component in an absorbent core; (2) exhibits reduced hysteresis as reflected by a low CDH:CAH ratio; (3) has a relatively low CDH value to allow other core components (e.g., storage components) having higher absorption pressures than the desorption pressure of the distribution foam to partition away liquid; (4) wicks liquid away from the acquisition zone of the absorbent core before the next liquid insult occurs; (5) is soft, flexible and comfortable to the wearer of the absorbent article; and (6) has a relatively high capacity for liquid so as to provide diapers and other absorbent articles that efficiently utilize core components.